Well, I've been using paypal and ebay for over 10 years now. 98% of those transactions went off without a hitch, but I will say that there were a few that didn't turn out. I couldn't say if it was due to the seller or things that were lost in the mail.
One day when you are able to use them, I recommend looking up the feedback of the seller themselves. If they have a good reputation, its much safer to make purchases from them than someone without good feedback, not matter what they're selling. If something does go wrong though, it isn't the fault of ebay, paypal or amazon themselves, as they even offer steps to work through disputes, or even offering refunds depending on the situation. You'll see